My top 3:
Saturday market day in Arles;
If car: Roussillon, Isle-sur-la-Sorge, St Remy drive;
Pont du Gard
I'm sure this may be sacrilege to some, but I would save Avignon for another trip. You could whisk through for a few hours and visit Palace of Popes, scenic roads, etc., but really Avignon is a place that you can either skip, or need to spend more time in.
No need for Aix, Nice, Marseilles (in 2.5 days, but with more time, yes, absolutely).
But all this depends on home base. If you are in the east, it makes more sense in 2.5 days to stay closer and visit nearby locations.
Base in one location, if using a car, use a hotel in a village or in the periphery of town or near municipal parking . Assuming you are flying into a major city, you will probably be taking the TGV to Avignon? If so, renting a car from there is easy, since it is on the outskirts of town. If you want to see more, consider a one-way rental and on your last day, drive to a different city.